Process for roasting ore



Sept. 29, 1942. u. s; LAUBER PROCESS FOR ROASTING ORE Filed Oct. 28, 1940 INVENTOR v Urban flauer y ATTORNEY Patented Sept. 29, 1942 Princess Fon aoAs'riNc oma Urban S. Lauber, GlenRoek, N. J., assignor to General Chemical Company, New York, N. Y., a corporation of New York Application october 2s, 1940, serial No. 363,137

` 7 Claims.

"I'his invention relates to roasting of metal lulde ore by suspension roasting methods particularly of the type described in Carter U. S. Patent 2,174,185 of September 26, 1939.

Much of the sulfur dioxide gas 'produced by roasting of metal sulfide ores such as pyrites is used in manufacture of sulfuric acid by the contact process. That portion of a contact sulfuric acid plant involved in practice of the present invention comprises the. following apparatus units: an ore roaster'for forming the raw S02 gas; hot purification apparatus such as dust chambers by means of which most of the solid particles entrained in the `raw SO2 gas are settled out; one or more so-called wet purification towers or chambers in which the gas stream is :scrubbed with water or weak/sulfuric acid lto remove residual solids and other` impurities and iorm a relatively cool gas stream of moderately low water-carrying capacity; coolers in which the gas stream is cooled ordinarily to temperatures of 10U-150 F. to condense out water vapor and sulfuric acid 'vapors and mists; and lters` such as coke boxes acting primarily to separate out of the gas stream sulfuric acid mist and impurities such as arsenic; all such apparatus units being associated together in the apparatus train in the order named. p The condensate formed in the .coolers following the wet scrub towers is largely water containing for example about 5% H2S04, while the condensate draining out of the coke boxes is a similar aqueous liquid having H2S04 concentration which may in some cases be as high as 20%. The liquid eilluent of the wet scrub towers also contains sulfuric acid, the H2804 concentration depending upon the particular type of scrubbing operastream and forms sulfates which, depending A upon the H2S04 strength of the liquors in -the purification system, are carried in the liquors in,

solution or as sulfate crystal particles in suspension. In' atypical operation, thev overall plant H2504 production is diminished 7% on account of H2504 and sulfate salt losses during purification.

In the earlier days of contact plant operation, multi-hearth furnaces'were used for ore roasting but because of the slow, relatively quiescent nature of hearth roasting the dust problem vcreated no special diiculty, and by means of one or two dust chambers it was possible to separate out of the roaster gas such a large proportion of the entrained dust that it was feasible to then wet-purify the gas in `packed scrub towers without plugging up the packing withY solids. In such a" scrub tower the gas stream usually was flowed upwardly countercurrent to a stream of weak sulfuric acid which included the 15-20% H2S04 drips from the' coolers and coke boxes. Since hearth roaster gas on entering the scrubbing towers was not dust-laden to a degree resulting in choking up the packing, and since in a packed tower it is possible to appreciably concentrate the H2804 content of the scrubbing liquid, practice was such that the 1 wet scrub towers were operated so that the sulfuric acid eiiluent of the scrub tower system had concentration of around 50 B., i. e., about `H2SO4. In this way puriiication acid was recoveredbut nevertheless in the form of a low strength, low-grade sulfuric acid for which there was and is very litle commercial demand. Further, at such H2S04 strength most of the .sulfate salts, predominantly iron, are precipitated out causing loss of S04 radical.

Following recent development of suspension roasting, hearth furnaces have been replaced by suspension or flash roasters. 0n acco t of the inherent characteristics of ash roasting, 'the dust problem became more acute because ,of the much greater quantities of dust carried in theV SO2,v gas leaving the suspension roaster. The

quantity of nely divided solids carried -in a v the former practice lof introducing into the top say 4 to 55 of the wet purification towers the weak acid'drip from the coolers and coke boxes was more or less abandoned, and water used as the scrubbing liquid. Thus, in hearth roaster practice purication system acid was recovered only as an impure weak e. g. 60% acid of little market demand, while when using suspension roasting prior to development of this invention, the strength of the purification acid was so low as to be of no practical value.

The principal object of this invention is to provide a method by practice of which it is possible not only to recover purication system H2804 and the S04 of associated sulfate salts but to effect such recovery in the form of plant strength sulfuric acid.

The nature of the invention will be understood from the following description taken in connection with the accompanying drawing showing diagrammatically apparatus which may be used. On the drawing, solid lines designate gas passage thru the several apparatus units and dot-and-dash lines the course of flow of liquids thru the system.

Referring to the drawing, I indicates a metal sulfide fines suspension roaster which may be constructed and, as will hereafter appear, is operated in accordance with the principles described in Carter U. S. Patent 2,174,185. The hot sulfur dioxide gas produced, at temperatures of e. g. 1650-1900 F. and having SO2 concentration of say -15%, passes successively thru line I2, preliminary gas cooler and dust collector I4, line I5, cyclone dust collector I'I, into line I9. In such apparatus, the gas isl ordinarily cooled down to around 1000 F. As known in the art, a waste heat boiler may be substituted for collector I1, in which case the temperature of the exit gas of the boiler may be considerably lower, e. g. GOO-'700 F. In any case, practically all of the heavier and most of the fine entrained solids are separated out of the gas stream. However, the proportion of such flne solids in the suspension lroaster gas is so great that even after the best feasible dry separation of dust the gas stream such as in line I9 still contains a substantial amount of dust referred to herein as residual solids. To illustrate, such a gas may carry 0.1 to 1.0 gram per cubic foot (N'IP) of dust, and priorto present improvements it was feasible to wet-purify such a gas only by the procedure previously described and involving practically a total loss of so-called purification acid and the S04 -of the contained sulfate salts. I have found a method by means of which such a gas may be wet-purified to the degree desired and in such a way that `all of the free H2SO4 and sulfate radical of sulfate salts formed during the roasting and purification operations may be recovered as plant strength sulfuric acid. Briefly stated, a prefer-red embodiment of the present invention comprises contacting the gas stream, such as in line I9 -and after dry separation therefrom of the bulk of entrained solids, in an unobstructed scrubbing tower with liquid in quantity and under conditions to cause separation from the gas streamof the major portion of the residual entrained solids, then contacting the thus partially scrubbed gas stream in a packed concentrating tower with a sulfuric acid solution, regulating the contacting operation in the packed tower so as to increase the H2SO4 strength of the solution, effect substantially complete removal of residual solids from the gas and so that the sulfuric acid liquor discharged from the packed tower is of concen- 1 tration not less than 30 and not more than 41 B., then introducing -at least a portion of the concentrating tower efiluent into preferably a metal sulfide fines suspension roasting zone and effecting 'decomposition of such portion to form SO2 by means of heat liberated in the suspension roasting operation as in accordance with the teachings of the Carter patent.

In the following description of the invention specific gas volumes, gas temperatures, dust contents and liquor concentrations mentioned are to be considered, except where otherwise indicated, as illustrative.

In the practice of the invention in a plant designed to handle say 4000 cubic feet per minute (NTP) of SO2 gas, such gas at temperature of about 1000 F. and carrying about 0.1 gram per cubic foot (NTP) of dust is charged into the top of an unobstructed scrub tower 2| which may be say twenty feet high and six feet in diameter and which is provided with a cone bottom 22 for liquor collection. Weak sulfuric acid solution of H2SO4 strength of say 10-20% (the source of such solution being hereafter described) is withdrawn from collecting tank 24 by pump 25 and fed into line 2'I terminating in one or more spray heads 30 designed to shower liquor over the cross-section of tower 2|. Liquor leaving tower 2| runs thru connection 32 into settler 33. 'I'he arrangement shown provides for passage thru tower 2| of a relatively large volume of liquor which may be fed in at temperature of 75 to 150 F. and leaves thru pipe '32 at temperature of about F15-225 F.

In tower 2| the hot SO2 gas is intimately contacted with scrubbing liquor. The chief functions of tower 2| are (1) to separate from the gas stream at least the major part and preferably 60% or more of the residual solids contained in the SO2 gas in line I9, and (2) to substantially cool the gas stream. Volume of the scrubbing liquor and temperature of the same entering top of tower 2| are variable and depend upon factors such as the size of the installation, volume of gas passing thru the system, solid material content of the SO2 gas in line I9, and also upon the particular gas stream temperature desired as the gas passes thru connection 34 and into the bottom of concentrating tower'35. In tower 35, sulfuric acid in the circulating liquor is concentrated, and in order to effect appreciable HzSO4 concentration, the gas entering tower 35 should .contain appreciable amount of heat and temperature should be preferably not less than 20o-275 F. Accordingly, taking into consideration the variables mentioned, and the primarily important feature of removing at least a major portion of solids from the gas-stream in tower 2|, and the secondary feature of cooling the gas to temperature not below that needed for appreciable acid concentration in tower 35, rate of feed of scrubbing liquid into tower 2| may be regulated by the operator in accordance with the particular conditions encountered. In the example under consideration, the gas was discharged from the tower thru connection 34 at a temperature of' about 250 F. The scrubbing acid at about 10% HzSO4 strength entered thru spray, 30 at temperature of about F. and was discharged thru pipe 32 at temperature of' about 225 F. at concentration of about 16 B. and had an H2SO4 `concentration of about 12% Liquid-passage over Operation of the wet purification system of this "invention, that is operation of scrubbing tower 2| and subsequent concentrating tower 35, is such sult is effected, asl subsequently discussed, by

maintaining concentration of the liquors in the purication system not in excess of 41 B., settled basis. In this specification and appended claims, mention of liquor concentration in terms of degrees Baum is intended to indicate liquor concentration after settling of substantially all solids normally suspended inthe liquor. In operation as described, 'concentration of the liquor eilluent of tower 2| does not normally exceed about 20 B., and should not be permitted to exceed 38 B. Any desirable concentration reduction may be effected by introduction of water from pipe 26. Minimum concentration of the liquor eflluent is not critical.

The liquor run-oil' of tower 2| comprises the H2SO4 initially present in the solution fed in at spray 30 plus H2504 picked up during passage thru the tower, such latter H2SO4 having been formed from S03 present in the S02 gas stream. Sulfuric acid 'concentration of the tower liquor eflluent does not exceed about 15% H2'SO4. The liquor contains solids and also sulfates of metals such as iron, zinc and lead, substantially all of which sulfatesare held in solution under the conditions indicated. The liquor runs thru pipe 32 into settler 33 which is 'large enough to facilitate deposition and settling out of practically all solids comprising mostly iron oxide, together with some silica.

Relatively clear sulfuric acid solution overflow ofthe settler, having H2804 strength normally of 15% or less, is run thru pipe 4|, controlled by Valve 4| into pump tank 42 at a rate controlled so that the composite liquor withdrawn therefrom by pump 43 -has a concentration of say 35- 38 B., preferably as high as feasible taking into consideration the degree of increase of concentration during the succeeding pass of liquor over tower 35. Such liquor is run thru line 45 to a spray head 41 in the top of concentrating tower by the downowing liquor. 0n account of increase in H2804 strength of the liquid phase of the liquor and entrainment oi' the solids scrubbed out of the gas, the concentration of the liquor during one pass thru the tower increases say 1 to 2 degrees B. The liquid phase of the liquid in tank 42 may have an H2SO4 strength varying from say 20 to 30%,` and although the amount of Vsolids in the gas entering tower is relatively small, the tower eilluent liquor may contain 0.1 to 0.2% by weight of solids. In cases where solid matter extraction in tower 35 is relatively high, part of the tower eilluent liquor may be by-passed continuously or intermittently thru pipe 5| around the pump tank 42 and into the settler 33.

I have found that, when the SO2 gas has been treated as previously described, i. e. to scrub out at least the major portion of residual solids contained in the gas stream on entering the wet puri'- cation system (e. g. tower 2|), and where the concentration of the effluent liquor of the tower 35 is kept within the 31-41 B. range, it is possible to remove from the gas the balance of the residual solids, effect an appreciable concentration of sulfuric acid, and further to keep in solution the sulfate salts of metals such as iron' (ferric and ferrous), zinc and lead, and at the same time avoid clogging of lpacking by solids. Further, I have discovered that in the case of a liquor of the kind described, the physical and chemical characteristics and properties are such that the 35. Temperature of the liquor as fed into tower the drawing with suitable packing 48 which effects relatively prolonged contact of gas with the liquid running down thru the packing. Part of the tower eilluent liquor, usually at temperature of 175 to 200 is returned thru pipe 48, header 49, and valve-controlled outlet to tank 42. Control of liquor circulation thru tower 35 is such that the concentration of the liquor in pipe 48 and header 49 is not less than 30 and" not more than 41 B. Ordinarily such concentration would be around 35-37 B. and rarely more than Gas entering the bottom of tower 35 passes upwardly thru packing 48 against downflow of "relatively thin lmsofsulfuric acid liquor distributed overthe top of the packing. YThe gas is cooled and leaves the tower thru conduit 53 at temperatures of 150 to 250'F. Wat-er is levaporated and H2804 strength of the liquor is inliquor may be treated in a suspension roaster, as hereafter indicated, to convert the sulfur values of the liquor to S02. `Moreover, I have observed that when concentration of the tower eluent is permitted to exceed 41 B., precipitation of sulfate salts takes place to such an extent as to risk plugging the tower or in any case precipitate sulfates and cause loss of S04 radicals of the same. On the other hand, the concentration of the tower eilluent should not be permitted to fall below 30 B. since the product then is not readily adaptable for handling in the suspension roaster. A secondary factor in operation of tower 35 is control of scrubbing liquorilow so that temperature of the gas leaving the tower is notl more than 300 F. and preferably less than 200 F.,

this latter feature being for the purpose of effecting such cooling of the gas as to utilize heat in the tower for acid concentration and to reduce water carrying capacity of the gas.

In the example under consideration, the S02 gas entered tower 35 at temperature of 250 F. and was discharged at 175 F; Liquor-:was fed into the top of the tower vat concentration of about 33 B. and temperature of 150 F., and the creased say 1 or 2% during one pass over the tower. "Substantially all remaining solid impurieiiluent liquor concentration was 40 B. and temperature 175 F. Liquor was circulated over the tower at rate of about 35 gallons per minute.

The SO2 gas then passes downwardly thru tubular cooler 56 the function of which is tocool the gas stream down close to F. so as to condense a large portion of the entrained water vapor and a substantial amount Iof sulfuric acid mist.I The condensate leaving cooler 56' thru is to be used in the contact sulfuric acid plant the gas is then owed thru pipe 66 to the usual drying system.

In lters 6I and 64 substantial amounts of acid mist are ltered out of the gas stream and drain thru outlets 'l0 and 1l into pipe 15 by means of which the drips of cooler 56 and of both coke filters are transferred to the collecting tank 24. i

The coke-box drips` ordinarily run around 20% H2SO4, and the composite condensate in pipe I5 may have H2SO4 concentration of 1015%, temperature ordinarily being 75-100 F. The greater part of the wet purification acid is recovered as condensate such as in pipe 15, the balance being present in the eiiluent liquids of towers 2| and 35.

uor recovered in header 49.

Strength at 60? F Be' 37.86 Free acid H2SO4 Per cent 32.03 Total sulfates as S04 do 35.84 Ferrous sulfate do 1.61 Ferrie4 sulfate do 1.68 Total iron as Fe do 1.06 Zinc sulfate do 4.40 Arsenic as As do .13 Total solids after ignition do 5.57

Part of the liquor eluent of tower 35, a quantityvcorresponding with the so-called acid make,

is run from header 49 thru pipe 16 to the suc-A tion side of pump 11 thence thru line 18 to one or more suitable spray nozzles 80 located for ex-` ample in the top of suspension roaster I0. Preferably, the liquor is passed thru settler 8| to remove any solids present `to lessen abrasion of the spray nozzles 80.

heat than is needed to maintain the oxidation reaction self-sustaining. present invention, the acid make of tower 35 is sprayed into the suspension roaster thru one or more suitably constructed and arranged nozzles, and by action of the high temperatures prevailing, e. g. 1600-1950 F., the liquor is. completely decomposed with respect tothe liquor thus fed into the suspension roaster, the reactions taking place are brieiiy as follows: Water is vaporized;`

free HzSOi is decomposed to Water vapor, oxygen,

and SO2; andv'metal sulfates, especiallyferrousA sulfate, are decomposed to form metal oxide cinder (e. g. FeaOi) SO2 and S03, the latter being immediately dissociated to SO2 on account of the high temperatures of suspension roasting. These principles are all more fully discussed in Carter U. S. Patent 2,174,185 to which reference is hereby made for detailed procedure as to how to operate a suspension roaster in such. a Way as to make use of excess heat for the decomposition of other materials to form sulfur dioxide and metal oxide cinder. Similar recovery of the sulfur values-of the purification system liquors may be had by spraying the liquor into the preliminary gas cooler |4. However, it is preferable to decompose the purification'acid in the burner proper Vto make most efficient use of the available heat A suspension roasting operation generates substantially greater quantities of In accordance with the.

and avoid loss of heat which might otherwise be dissipated as radiated heat.

The suspension roaster shown in the Carter vpatent operates on the so-called co-crrent principle, that is the material to be roasted and the oxidizing gas passing thru the combustion zone in the same direction. Freeman U. S. Patent 2,030,627 of February 11, 1936, discloses a countercurrent suspension roasting method, and Mullen U. S. Patent 2,070,236 of February 9. 1937. describes a third type of operation. As far as practice of this invention is concerned, it is not important'which type of suspension roasting is used. Acid make of tower 35 may be decomposed in conjunction with the Freeman or Mullen or any similar suspension roasting method. Regardless of the physical nature of the suspension roasting operation-whether co-current as in the Carter patent or countercurrent as in the Freeman patent or otherwise--it will be understood that the roasting operation, as far as the decomposition of the acid make of tower 35 is concerned, is carried out in accordance with the teachings of the Carter patent. All that is needed to carry out the last phase of the process of this invention is the provision on known suspension burners of a suitable number of liquor injection nozzles preferably located immediately adjacent the known sulfide fines feed mechanism.

In the operation of a plant such as described, it is noted that in 4summer ordinarily more water vapor goes into the drying apparatus thru pipe 66 than enters the system as moisture in the air and sulde fines fed into burner I0. Hence, any water loss may be balanced by water introduced thru pipe 26. In winter, usually less water passes to the drying unit than enters the burner with the air and ore. Thus, should water accumulate to an undesirable degree in the system shown on the drawing, some of the liquor may be bled out at any convenient point. Generally, however. it

f may be considered that the water content of the liquors circulates continuously thru the burner' and puification units.

The SO2 formed in the suspension roaster by decomposition of the H2804 and metal sulfates of the overflow of tank 42 mingles with the SO2 gas produced by burning of the sulde fines.A and all of the sulfur values in the system eventually pass thru pipe 66 as SO2 for conversion in the contact plant, not shown, to commercial strength sulfuric acid. f

I claim: Y g

1. In the production and purification of SO2 gas by an operation involving oxidation of metal sulfide fines in a suspension roasting zonewith formation of metal oxide cinder and a hot SO2H gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulk of entrained solids, and subjection of the gas to wet purification to effect separation of residual solids and other impurities, the improvement comprising contacting the gas stream, after dry separation therefrom of the bulk' of entrained centrating chamber with a sulfuric acid solution,

regulating the contacting operation in said packed chamber so as to increase H2804 strength of said solution, to'effect substantially complete removal of residual solids from the gas and so of said packed chamber has concentration not` less than 30 and not more than 41 B., introducing at least a portion of said liquid eiiluent in to said suspension roasting zone, and dissociating said portion to form SO: by means of heat liberated on suspension roasting of further quantities of metal sulde fines.

2. In the production and purification of SO2 gas by an operation involving oxidation of metal sulde nes in a suspension roasting zone with formation of metal oxide cinder and a hot SO2 gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulk of entrained solids, and subjection of the gas to wet purification to eiect separation of residual solids and other impurities, the improvement comprising contacting the gas stream, after dry separation therefrom of the bulk of entrained solids, in an unobstructed scrubbing chamber with liquid in quantity and under conditions to cause separation from the gas stream of .the major portion of said residual solids entrained in the gas stream entering said scrubbing chamber, contacting the gas stream in a packed concentrating chamber with a countercurrent flow of a sulfuric acid solution, regulating now of liquid and g'as in said packed chamber so as to increase H2804 strength of said solution, to effect substantially complete removal of residual solids from the gas, to cool gaseous eiiiuent of such chamber to not more than 300"v F., and so that the sulfuric acid containing liquid eluent of said packed chamber has concentration not less than 30 and not more than 41 B., introducing atleast a portion of said liquid eiiluent into said suspension roasting zone, and dissociating said portion to form SO2 by means of heat liberated on suspension roasting of further quantities of metal suliide nes.

3. In the production and purification of S02 gas by an operation involving oxidation of metal sulfide fines in a suspension roasting zone with formation of metal oxide cinder and a hot SO2 gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulk of entrained solids, and subjection of the gas to wet purification to effect separation of residual solids and other impurities, the improvement comprising contacting the gas stream, after dry separation therefrom of thebulk of entrained solids, in an unobstructed scrubbing chamber with sulfuric acid solution in quantity and under conditions to cause separation from the gas stream of the major portion of said residual solids entrained in the gas stream entering said scrubbing chamber, contacting the gas stream in a packed concentrating chamber with a sulfuric acid solution, regulating the contacting operation in said packed chamber so as to cool gaseous eiuent thereof to not more than 300 F. and so that the sulfuric acid containing liquid eiliuent of said packed chamber has concentration not less than 30 and not more than' 41 B., further cooling the gas stream to condense sulfuric acid vapor and mist, passing the resulting condensate gas by an operationinvolving oin'dation of metal sulde nes in a suspension roasting zone with formation of metal oxide cinder and a hot SO: gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulk of entrained solids, and subjection of the gas to Awet purification to effect separation of residual solids and other' impurities, the improvement comprising contacting the gas stream, after dry separation therefrom of the bulk of entrained solids, in an unobstructed scrubbing chamber with liquid in quantity and under conditions to cause separation from the gas stream of the `major portion of said residual solids entrained in the gas stream entering said scrubbing chamber, contacting the gas stream in a packed concentrating chamber with a sulfuric acid solution, regulating the contacting operation in said packed chamber so asto increase HnSO4 strength'of said solution, to effect substantially complete removal of residual solids from the gas and so that the sulfuric acid containing liquid eilluent of said packed chamber has concentration not less than 30 and not more than 41 B., and'then dissociating at least a portion of said liquid eilluent by means of heat liberated on suspension roasting of further quantities of metal sulide fines to thereby recover, as SO2, sulfur values contained in said portion.

5. In the production and purification of SO2 gas by an operation involving oxidation of metal sulde'nes in a suspension roasting zone With formation of metalfoxide cinder and a hot SO2 gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulkof entrained solids, and subjection of the gas to wet purification to eiect separation of residual solids and other impurities, the improvement comprising treating the gas stream, after dry separation therefrom of the bulk of entrained solids, to eil'ect separation from the gas stream of the major portion of said residual entrained solids, contacting the resulting gas stream in a packed concentrating chamber with a sulfuric acid solution, regulating the contacting operation so as to increase H2804 strength of said solution and so that the liquid eluent of such contacting operation has an HiSOr concentration not in excess of that causing precipitation of contained metal sulfate, and then dissociating at least a portion of said liquid effluent by means rst thru the scrubbing chamber and then, after separation of solids, thru the concentrating chamber, introducing at least a portion of the liquid eiliuent of the concentrating chamber into said suspensionroasting zone, and dissociating said portion to form SO2 by means of heat liberated on ,uspension roasting of further quantities of metal sulfide fines.

4. In the production and purification of SO:

of heat liberated ori suspension roasting of further quantities of metal sulfide fines to thereby recover, as SO2, sulfur values contained insaid portion.

6. In the production and purification of SO2 gas by an operation involving oxidation of metal sulfide fines in a suspension roasting zone with formation of metal oxide cinder and a hot S02 gas stream carrying entrained solids, separation from the gas stream in the dry way of the bulk of entrained solids, and subjection of the gas to wet. purification to effect separation of residual solidsiand other impurities, the improvement comprising treating the gas stream, after dry separation therefrom of the bulk of en- -plete removal of residual solids from the gas and so that the sulfuric acid containing liquid emuent of such contacting operation has concentration not less than 30 and not more than 41 B., introducing at least a. portion of said liquid eiiluent into said suspension roasting zone, and

l dissociating said portion toform SO2 by means of heat liberated on suspension roasting of further quantities of metal sulde ilnes.

7. In the production and purication 'of SO2 gas by an operation involving oxidation of metal sulfide nes in a suspension roasting zone with formation of metal oxide cinder'and a hot SO2 gas stream carrying entrained solids, separation from the gas stream of the bulk of entrained solids, and subjection of the gas to wet purication to effect vseparation of residual solids and other impurities, the improvement comprising scrubbing the gas stream with aqueous liquid in quantity and under conditions to cause separation from the gas stream of the major portion of solids entrained in thel gas stream as subjected to said scrubbing, contacting the resulting gas stream with a sulfuric acid solution, regulatine,r the contacting operation so as to eiect substantially complete removal of residual solids from the gas and so that the sulfuric acid containing liquid eiiiuent of such contacting operation has concentration not less than 30 and not more than 41 B., introducing at least a portion oi' said liquid eiiluent into said suspension roasting zone, and dissociating said portion to form SO2 by means of heat-liberated onsuspension roasting of further quantities of metal sulfide fines.

URBAN S. LAIJBER. 

